This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 10(1998)-357087 filed on Dec. 16, 1998, the entire content of which is incorporated herein by reference.
The present invention generally relates to a negative pressure type servo unit. More particularly, the present invention pertains to a negative pressure type servo unit applied to an automobile.
Japanese Patent Laid-Open Publication No. 2246/1997 describes a negative pressure type servo unit that includes a housing forming at least one pressure space within its interior, a movable wall installed in the housing progressively and regressively movable relative to the housing and dividing the pressure space into a front chamber communicating with a negative pressure source and a rear chamber selectively communicated with the front chamber and the atmosphere, a power piston coupled to the movable wall, an input member arranged at an inner portion of the power piston and movable in a forward and rearward direction by operating a brake member, and a plunger arranged coaxially within the power piston and movable in the forward and rearward direction through movement of the input member. A seal member is arranged between the plunger and the input member, and a valve mechanism is provided for communicating the rear chamber with the atmosphere and cutting off the rear chamber from the atmosphere and for communicating the negative pressure source with the rear chamber and cutting off the negative pressure chamber from the rear chamber by moving the plunger. An output member outputs the progressive force of the power piston to outside the unit by being moved through movement of the power piston in accordance with movement of the movable wall. An actuator is also provided for moving the plunger. A pressure difference is produced between the front side and the rear side of the seal member at least when the front chamber is communicated with the rear chamber and the plunger is slidably moved on the seal member.
In the above-described negative pressure type servo unit, the pressure difference is produced between the front side and the rear side of the seal member at least when the front chamber is communicated with the rear chamber and accordingly, there is a concern that a sticking phenomenon may occur in which a sliding portion of the seal member where the plunger slidably moves sticks to the plunger by the pressure difference.
When such a sticking phenomenon occurs in the seal member, the drive force of the actuator must be increased and this may lead to the need for a relatively large actuator.
A need thus exists for a negative pressure type servo unit that is not as susceptible to the aforementioned sticking phenomenon and that permits the size of the actuator to be reduced.
In light of the foregoing, the present invention provides a negative pressure type servo unit that includes a housing in which is defined at least one pressure space, a movable wall installed in the housing for progressing and regressing relative to the housing and dividing the pressure space into a front chamber communicated with a negative pressure source and a rear chamber selectively communicated with the front chamber and atmosphere, a power piston coupled to the movable wall, an input member arranged at an inner portion of the power piston and movable in a forward and rearward direction by operating a brake, a plunger arranged to the input member coaxially inside the power piston and movable in the forward and rearward direction by moving the input member, a seal member arranged between the plunger and the input member, and a valve mechanism for communicating the rear chamber with the atmosphere and cutting off the rear chamber from the atmosphere and communicating the negative pressure source with the rear chamber and cutting off the negative pressure chamber from the rear chamber by moving the plunger. An output member outputs a progressive force of the power piston to outside the unit by being made to progress as a result of progress of the power piston in accordance with movement of the movable wall, and an actuator is adapted to move the plunger. The seal member includes a first engaging portion engaged with the input member so that the engaging portion is movable integrally with the input member and a second engaging portion engaged with the plunger so that the second engaging portion is movable integrally with the plunger.
According to another aspect of the invention, a negative pressure type servo unit includes a housing in which is formed at least one pressure space, a movable wall installed in the housing for progressing and regressing relative to the housing and dividing the pressure space into a front chamber communicated with a negative pressure source and a rear chamber selectively communicated with the front chamber and atmosphere, a power piston coupled to the movable wall, an input member arranged at an inner portion of the power piston and movable in a forward and rearward direction by operating a brake, and a plunger arranged at the input member inside the power piston and movable in the forward and rearward direction. A seal member is arranged between the plunger and the input member, and includes a portion engaging the plunger to move integrally with the plunger. A valve mechanism controls relative pressures in the front and rear chambers, and an output member outputs a progressive force of the power piston to outside the unit by being made to progress as a result of progress of the power piston in accordance with movement of the movable wall. The unit also includes an actuator adapted to move the plunger.